The device disclosed herein is adapted to function with automatic radio telephone (ART) systems which are tied directly into telephone transmission networks. ART systems permit a user to place a call by direct dialing via a standard telephone or radio telephone and reach a standard telephone station or radio telephone station. If the number dialed is a radio telephone station, the area code routes the call in the standard way and the exchange code identifies a radio telephone terminal which transmits the last four digits of the dialed number as a user identification code. In some systems, the area code identifies a radio telephone terminal which transmits the last seven digits of the dialed number as a user identification code. This code, either four or seven digits, is in the form of a pulsed 2805 HZ tone on a radio telephone channel carrier frequency.
Radio telephone receivers tuned to the channel decode the pulses and if they represent the proper telephone number, a ring tone is generated. The 2805 HZ tone remains on the channel carrier until the telephone call is terminated. After being pulsed and providing the dialing code, the tone remains constant and functions as a busy tone which blocks all other channel users except the one which received the call. The channel is protected against other stations transmitting as well as receiving to provide privacy on used channels. Notch filters are used in the audio section of the system to mask or block out the 2805 HZ busy tone so that it does not interfere with normal use of the radio telephone link completed between stations.
The radio telephone station which decodes the number sends an identification coded signal back to the calling station to indicate that the number has been decoded by the party called. The signal is decoded by the call originating station which then locks up the channel until the call is terminated as signaled by a positive disconnect code signal. Upon receiving a positive disconnect signal, the system is automatically shut down to free the channel for other uses.
One ART system for which the present invention is adapted to function in conjunction with is the Secode Modular Automatic Radio Telephone System provided by Secode Electronics of Dallas, Tex. A number of encoder/decoder systems are available to function in conjunction with that system as well as other ART systems but they all create excessive power drain on battery systems providing power to associated mobile receiver transmitters.
In addition to creating excessive power drains, existing encoder/decoders require full duplex operation, do not provide audio or visual ques to an operator to indicate when a channel has been secured, when keyboard entries are being made, when a channel is busy or when a station has been called.